Ol' Dirty started out as a feedback experiment with an LM386 which ended up as a
dirty fuzz/distortion effect. No calculations where involved in the design of the circuit, just some experimental breadboarding.

I added the light because it just happened to lay on my table (left over from my dancing santa) and I don't think it really effects the sound but it looks great.
The old steel box I used is something I had welded a very looong time ago, and I used nuts as knobs.

more information, photos and maybe more samples will be added to my website, but for know here's the schematic, a picture and a sample. (some of you will probably recognize the tune )

of course feel free to ask any questions,..

edit: I noticed that I had posted the (old) schematic that didn't have the switches in it. So I changed it to the version with switches.

The circuit has an input level control pot. followed by an inverting amp (U1). And an inverting output amp (U3) which mixes the original sound with the distorted sound, followed by a volume control pot. In between these two amps sits the main distortion/fuzz/feedback circuit build around the LM386 (U2). The 100K pot. controls the feedback level and you can set it from little distortion, bursts of oscillation noise, up to a point where the LM386 goes in to complete self oscillation. The 4K7 pot is the gain control and it controls the amount of fuzzyness. The diodes are used for hard clipping to create the fuzzy sound and they also make sure that the ouput level doesn't get too high.

of course i have some n00b questions
i guess this isn't just for line level input so i could also put in say a bass or guitar?
if i like to waive the lamp can i just leave it out together with the 10ohms resistor?

of course i have some n00b questions
i guess this isn't just for line level input so i could also put in say a bass or guitar?

Well, unfortunaly i don't have a guitar myself so I can't test it. Usually guitar circuits have a higher input resistance (about 1M) and I think a guitar level is also very low so you need more gain. Therefore you would probably have to adjust the input circuit a bit. (you could also put an extra opamp before the input, or you could try this circuit)

I don't know if there's a volume control on the guitar but if there is the following might work: discard the 4.7uF cap. and 47K pot, change the 1uF to a 100nF and connect the input with a 1M resistor to GND. Change the 47K (connected to pin 2 of U1) to 220K and the 100K (connected between pin 2 & 6 of U1) to 1M. Of course you just have to test it for yourself to see what works, maybe you don't even have to change anything at all.

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if i like to waive the lamp can i just leave it out together with the 10ohms resistor?

Well that's what i thought, so I just tried it to be sure and it does make a big difference. I did some more experimenting and you could replace the '10ohm + lamp' with a 22ohms resister (0.5W or maybe 1W) but it doesn't sound the same. So I would suggest using a lamp, which also functions as a very nice indicator.

I just fried the original lamp while I was testing , but I do have some similair lamps. They are not exactly the same so the sound is slightly different now. The new lamp came from a 3V scanradio/flashlight. Since I don't have anymore data of it, I measured the current at 3V and it's about 200mA. I think you need something like this: http://www.conrad.nl/goto.php?artikel=727164 .

If you're not discouraged and do decide to build it I would love to see/hear the result!

thanks for your explanations. i got the parts & will try it out. i already have a similar lamp, looks exactly like the one from Conrad, is also from Conrad but has 12V instead of 3V.
as such lamps are a ohm resistive load i thought i could also try a potentiometer i have instead of the lamp, if the correct functioning of the circuit depends on that power dissipation. it's a 10K 4W wire type. i'm curious how (if) that works but will try both...
but i'll rather not power it from batteries then (i guess).

thanks also for the link to the stompbox thread- and sorry you fried your lamp!
i'll try to record some sounds then, too (haven't done this with a computer yet)

thanks for your explanations. i got the parts & will try it out. i already have a similar lamp, looks exactly like the one from Conrad, is also from Conrad but has 12V instead of 3V.

a 12V lamp will probably work though the sound will be different, but maybe it's for the better, . I tried a 6V lamp (0.6W, bicycle rearlight) myself and I noticed that because of the larger resistance the oscillation frequency was lower, which makes sense.

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as such lamps are a ohm resistive load i thought i could also try a potentiometer i have instead of the lamp, if the correct functioning of the circuit depends on that power dissipation. it's a 10K 4W wire type. i'm curious how (if) that works but will try both...

I think the big difference with a resistor is the fact that the resistance of a lamp changes because of the amount of current flowing through it. This gives it a better/warmer? sound which you don't get with a resistor. So if you use a potentiometer you will be able to control the oscillation frequency but you won't get the same sound. 10K is probably way too big since I got an acceptable sound with a total resistance of only 22 ohms. But just try it, and see what it does. Though I would like to suggest using a series resistor while testing so you don't accidentaly burn the potentiometer or maybe the LM386.
I think a potentiometer with a low resistance in series with a lamp could actually be a very nice feature (or maybe different lamps and a switch)

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but i'll rather not power it from batteries then (i guess).

No, you're right. Batteries will work but as you probably already guessed, they wouldn't last very long.

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